Zinc oxide (ZnO) is a material of a wurzite crystal structure having a hexagonal structure, which is a direct transition type semiconductor material having a wide band gap of 3.32 eV at a room temperature and can be implemented to have features of a transparent object or the like with respect to semiconductor, piezo-electricity, ferroelectric substance, ferromagnetic substance, conductor, visible light and the like depending on a doping material.
In addition, a zinc oxide thin film may be advantageously grown on various types of substrates. Accordingly, a variety of devices can be manufactured by attaching a semiconductor film such as silicon, a metal film such as platinum or a transparent conductive film such as ITO on the zinc oxide thin film.
The zinc oxide used for an electronic device is generally used in the form of a thin film. However, recently, use of a nano rod or a nano wire having a nano structure is increased. The zinc oxide has a polar semiconductor structure, and since the surface is finished with a material of either zinc or oxygen with respect to (0002) plane, stress energy of the surface is different on each (0002) plane, and since the growth rate on the c-axis is larger than those on the other axes due to such a crystallographic characteristic, a nano structure of a nano rod or nano wire form can be grown using this feature.
Studies on applying such a zinc oxide nano structure to a solar cell are actively under progress. It is since that if the zinc oxide nano structure is used, the surface area of a solar cell may be extended greatly, and efficiency of the solar cell may be improved by reducing moving distance of electrons and holes generated through optical absorption and preventing recombination of the electrons and holes.
In a typical method of fabricating a zinc oxide nano structure applied to a solar cell, first, nano dots of Au or an Au—Zn alloy are formed on a substrate as a seed of a zinc oxide nano wire, and then the zinc oxide is grown thereon by applying a vapor-liquid-solid (VLS) or vapor-solid (VS) growth method.
Such a conventional method of forming a nano structure may not adjust thickness of the nano structure and uses expensive Au in the process of forming a seed layer.
Since the zinc oxide nano wire has a large aspect ratio, it is difficult to uniformly deposit a thin film on the surface of the zinc oxide nano wire, and since the zinc oxide nano wire has a weak strength, it is difficult to be manufactured as a device such as a solar cell.
In addition, the holes and electrons generated in the solar cell are difficult to move smoothly since the nano wire is thin and thus has a large resistance, and since doping materials for forming the nano wire are limited and the amount of a doping material is difficult to precisely adjust, electrical conductivity cannot be improved through the doping.
Furthermore, since a growth direction of the zinc oxide nano wire cannot be controlled, the nano wire is difficult to be manufactured as a device such as a solar cell if the nano wire does not grow uniformly and vertically. Particularly, although an electrolyte or an organic material should penetrate between the nano wires in the case of a dye-sensitized solar cell or an organic solar cell, if the nano wire does not grow uniformly and vertically, the electrolyte or the organic material may not uniformly penetrate between the nano wires.
Korean Patent Reg. No. 10-1040956 discloses a technique of applying a zinc oxide nano wire structure to a thin film silicon solar cell. This patent relates to a method of manufacturing a thin film silicon solar cell on a zinc oxide nano wire grown in a solution of zinc nitrate hydrate and hexamethylenetetramine (HMT), without using an expensive seed such as Au.
Although such a conventional technique is improved in that an expensive seed is not used, it does not solve the problem of permeability and resistance related to the aspect ratio.